According to a known fluid pump disclosed in JP2008-169763A, driving power of an engine is transmitted by means of an accessory driving belt wound around a driving pulley. Thus, as the driving pulley rotates, a rotary shaft rotates. When the rotary shaft rotates, an impeller rotates unitarily with the rotary shaft, thereby circulating a cooling water through an engine cooling system.
According to a variable displacement fluid pump disclosed in JP2009-293578A, effective heights of vanes are changed by means of a movement of a movable member. According to the variable displacement fluid pump, the effective heights of the vanes are changed on the basis of a change in a volume of a thermo wax which is caused by a heat transmission, thereby varying a discharge amount of the variable displacement fluid pump.
However, the fluid pump disclosed in JP2008-169763A always circulates the cooling water when the engine is running. This may restrict a reduction in a warm-up time of the engine because the cooling water is circulated even during a start-up of the engine.
The variable displacement fluid pump disclosed in JP2009-293578A always operates even when the effective heights of the vanes are at the minimum height, thereby generating a driving torque. This may result in a wasted consumption of power.
A need thus exists for a fluid pump which is not susceptible to the drawback mentioned above.